1. Field of the Invention
The present invention relates generally to plasma welding torches and, more particularly, but not by way of limitation to plasma welding torches of the transferred arc type.
2. Brief Description of the Prior Art
As is common knowledge, welding is one of the basic tools that have been developed for working metal and is used in a large number of practical applications. For example, it is commonly known to join two pieces of metal by means of welding torches which heat the metal either by chemical or electrical means and, similarly, welding can be used to hard-face metal objects. Moreover, welding torches are commonly employed in metal cutting operation.
Because of the wide range of application of welding to various metal forming operations, a considerable variety of welding torches, having differeing operating characteristics and differing capabilities, have been developed. A particularly useful type of torch for many applications is the type which produces a plasma electrically to heat a work piece and/or materials which might be fed into the plasma to be added to the work piece. As noted in U.S. Pat. No. 3,148,263 issued Sept. 8, 1964 to Jensen, the plasma produced by such a torch will often reach temperatures in the range of 10,000.degree. F. to 35,000.degree. F. so that the torch can be used to melt alloys which have very high melting points. Moreover, the plasma is capable of carrying very high electrical currents so that plasma torches can be constructed to very rapidly and efficiently perform a variety of jobs. For example, it is not uncommon to construct plasma torches to operate with currents of thousands of amperes passing through the plasma produced thereby.
A basic problem with plasma torches is that, because of the temperture of the plasma and the current carried thereby, the plasma can severely erode electrodes by means of which current is supplied to the plasma with the result that the torch has only a very limited operating lifetime. A solution to this problem, which can be employed in many circumstances, is to use a transferred arc scheme of operation. In such a scheme, a central electrode, which can be made of a refractory material such as thoriated tungsten and which can be mounted in the torch to permit replacement thereof, is positioned along the axis of a second electrode which is tubular in form. In use, an arc is first established between the two electrodes to initially form a plasma in a gas that flows therebetween and the arc is then transferred to the work piece; that is, a main current carried by the torch is established between the central electrode and the work piece. Since the central electrode is formed of a refractory material that can be replaced and since the tubular electrode is not subjected to erosive effects that would occur where the main current might be passed thereto, such a torch will have a substantially indefinite lifetime.
An important consideration in constructing a transferred arc torch is that the alignment between the electrodes be maintained in a substantially coaxial relation despite heating of portions of the torch near the plasma, such heating occurring when the torch is used. Should the two electrodes become misaligned, the main arc can jump back to the tubular electrode with the result that severe erosion of such electrode can occur. Indeed, such erosion, if permitted to continue for even a relatively short time, can result in extensive damage to, or even destruction of, the tubular electrode.
The need for maintaining the alignment of the two electrodes in a transferred arc type plasma torch and thermal expansion effects, which tend to disrupt such alignment, has generally resulted in limitations on the applications of torches of this type or difficulties in their use. A particular problem area has been in the use of transferred arc welding torches in applications in which the plasma jet issuing therefrom must be directed against the wall of a confined space. Torches which direct the jet axially of an elongated member which can be inserted into the space, while generally not subject to electrode misalignment problems, often do not permit the plasma to be properly directed within the space for the purpose at hand. While the problem can be overcome, without introducing electrode alignment problems, by mounting a welding head on an elongated structure, such welding heads tend to be bulky so that limits are placed on the size of the space that the torch can enter. Thus, prior to the present invention, it has generally not been possible to effectively employ transferred arc plasma welding in many applications wherein welding is to be done inside a small space; for example, where portions of the inside of a valve are to be hard-faced, an application for which transferred plasma welding would be particularly appropriate, in the absence of the above problems, because of the temperature and current capabilities available in this type of welding process.